Each chloroplast has two outer membranes, which encloses a semi-fluid interior called the stroma. I inside the stroma is a third membrane called the thylakoid membrane. It located in the thylakoid membrane of the chloroplast. It is the light-independent reactions take place. The thylakoid membrane absorbs photon energy of different wavelengths of light. Embedded in the thylakoid membranes are photosystems: Clusters of 200 to 300 pigments and other molecules that surround a pair of chlorophyll a molecules. In plants, there are two photosystems, Photosystem I and Photosystem II. The light-dependent reactions begin in photosystem II.
When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains a higher energy level.
The begins with Photosystem II, where trapped light energy is used to split water, a process known as photolysis: H2O → 2H+ + 2e−+ O. It located in the thylakoid membrane. The electron is transferred from one to another molecule and creates a chain of redox reactions. It is an electron transport chain (ETC).
Along the way, the energy released by the electrons is used to make ATP in a process called photophosphorylation. The molecular details of this ATP-generating system are similar to those used by the mitochondrion in oxidative phosphorylation. Phosphorylation refers to the addition of a phosphate group ADP to form ATP. The reaction sequence is: ADP + Pi → ATP. The electron flow goes from PSII to cytochrome b6f to PSI. In PSI, the electron gets the energy from another photon. Photosystem I also traps light energy, and uses it to excite electrons along a series of carrier molecules. Combined with the H+ ions formed in Photosystem I, they react with NADP to produce reduced NADP. That is the NADPH:
NADP + 2H+ + 2e− → reduced NADP, Therefore, the final electron acceptor is NADP.